In Borderlands 1 the second playthrough had a "playthrough 2.5" once you beat The Destroyer, which changed the game so enemies (and most mission rewards) scaled to your level. Because of this it was best to play only story missions until beating the Destroyer so mission loot would be the best loot possible.

How do the playthroughs work in Borderlands 2? I've unlocked True Vault Hunter mode which starts out just like playthrough 2 in BL1, but I don't know if there's a "2.5" equivalent of if it's finally a real third playthrough or what.

3 Answers
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The system is mostly the same as it was in Borderlands 1: Beating playthrough one does nothing except unlock playthrough two ("True Vault Hunter Mode"), beating playthrough 2 scales enemies and rewards to your level, unlocking playthrough 3 (if you're level 50, I believe). Playthrough 3 is new and scales to your level from the start.

True Vault Hunter Mode (playthrough 2-2.5)

Once you beat playthrough two, enemies scale to within 3 levels of you; some weak enemies (like Skag Pups) may be level 48, most enemies will be level 49-51, bosses will often be level 52, and occasionally enemies will even be 53. This is known as "playthrough 2.5" as it was in Borderlands 1.

Loot scales to within a couple levels; most drops will be level 49 or 50. Quest rewards scale to your level too; this is why it's important not to start certain side quests until playthrough 2.5, otherwise you'll miss out on the highest level rewards. Side quests scale to when you start the mission so don't take them too early, if you want a level 50 reward, wait until you've finished the main quest. Of course if the reward is green, you can go ahead and do it because the reward is junk anyway.

Note, without the UVHM DLC that it's still important to cap out your level, not just get to playthrough 2.5 before getting rewards, but as of right now the max level without the UVHM DLC is still 50, and it's hard not to be level 50 by the time you complete the second playthrough.

Ultimate Vault Hunter Mode (playthrough 3)

As of a patch April 2013, there is also now an Ultimate Vault Hunter Mode or playthrough 3, which is scaled to your level and much harder. It's mostly explained in these patch notes and this explanation:

Ultimate Vault Hunter Mode is unlocked for a character once they have completed the main story missions in True Vault Hunter Mode and reached level 50.

Unlike other playthroughs, Ultimate Vault Hunter Mode can be replayed multiple times with players able to reset their overall mission progress at any time from the Main Menu.

No more tutorial missions -- characters in Ultimate Vault Hunter Mode begin in Southern Shelf with the "Cleaning Up the Berg" mission.

While playing in Ultimate Vault Hunter Mode, enemies and bosses will scale according to the player's current level or, if playing with others, the highest-level player in that party.
-Gameplay changes in UVHM:

Enemy health generally increased 4x.

Enemies now have a moderate amount of health regeneration.

Increased duration of slag damage multiplier effect.

Upped the damage that slagged enemies take from 2x to 3x.

Weapon swap speed increased to better facilitate slag use.

Enemies now more likely to drop ammo.

Loot Midgets are now "Legendary Loot Midgets" that can drop Legendary and other top-tier gear.

Powerful new “Ancient” E-Tech relics and rare Pearlscent-grade weapons can be picked up in Ultimate Vault Hunter Mode.

Generally speaking, enemies are harder, have much more health, slag is much more effective, rare drops are increased and certain new E-Tech relics and Pearlescent (cyan colored) equips are avalible only in UVHM. The E-tech relics and Pearlescents are only availible in UVHM for people with the Ultimate Vault Hunter Upgrade pack ($5 DLC, included for free with the Season Pass).

Note that due to the increased difficulty and increased efficacy of Slag, UVHM is particularly suited to co-op play. Just like in "raid" boss fights, a Siren with Res is always useful.

To my knowledge there's no "playthrough 3.5" since the ".5" was always about enemies/loot that scale to your level. Playthrough 3 scales from the start so there's no extra post-playthrough for scaling.

Should you play TVHM then? Or get to 50 in Normal before swapping over? I've unlocked it with my lv34 Soldier, but am still playing Normal.
– David YellJan 14 '13 at 14:22

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@DavidYell oh definitely play TVHM, grinding to 50 in Normal would be painful and then TVHM would be stupid easy until the very end. Just avoid the sidequests that give blue/better items, the story quests are good to go
– Ben BrockaJan 14 '13 at 14:23

there is a third playthrough now, see my answer.
– KevinApr 3 '13 at 15:49

The marked answer was correct, but it is no longer as per April 2 2013, when the newest addition to the game came out. The level cap is now raised to 61, and an additional playthrough is possible after beating TVHM and reaching level 50 (ultimate vault hunter mode). In this new playthrough, all monsters scale according to the player's current level, and you can reset your story progression whenever you want. You can also carry more eridium (500) and there are 2 more upgrades per category to spend it on. Pearlescent weapons are added, and enemies are a lot stronger.

As far as I understand, the 2.5 mode is still present, it's just not useful anymore because of the raised level cap and the added new playthrough.

If you have the Ultimate Vault Hunter Upgrade Pack, will enemies in TVHM / Playthrough 2 hit a level cap at around level 50, or will they also scale with you until level 61?
– galacticninjaJun 26 '13 at 11:07

There is a 2.5 equivalent. After finishing True Vault Hunter mode (just like the first playthough except that you start it with your level at the point of finishing the first playthrough, some enemies are stronger and renamed) all enemies scale to your level (50 at that point). This doesn't mean all enemies are 50, some are programmed to always be x levels below or above you. There sadly is no third playthrough.

Source: Visited the southern shelf where you start the game, everyone is level 50.

The good part is that the loot also (somewhat) adapts, meaning better weapons everywhere.

Are quest rewards included in the level scaling? e.g. a level 35 quest will then have a level 50 (or at least 45+) reward at playthrough 2.5?
– Ben BrockaSep 30 '12 at 16:10

Not 100% sure yet, will update post then. I think they do scale, but normal quests are still much better than trivial quests. They might also stay at the level. I'll check soon.
– SchottliciousOct 1 '12 at 1:26

there is a third playthrough now, see my answer.
– KevinApr 3 '13 at 15:48